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The impact of blossom and postbloom thinners on fruit set and fruit quality in apples and stone fruits

Fallahi, E., Greene, D.W.
Acta horticulturae 2010 no.884 pp. 179-187
ammonium thiosulfate, apples, benzyladenine, bud initiation, carbaryl, cultivars, deblossoming, endothal, ethephon, fish oils, fruit drop, fruit quality, fruit set, hydrogen cyanamide, nectarines, peaches, pelargonic acid, plums, sulfur, temperature, trees, weather
Early thinning of apples and stone fruit is important because of its impact on fruit size and next season’s flower bud initiation, but hand-thinning is a costly practice and an unacceptable alternative. Since 1989, researchers have been searching for a suitable blossom thinner to replace Elgetol. To achieve this goal, we have studied the effects of various rates and/or timings hydrogen cyanamide (Dormex), sulfcarbamide (Wilthin), pelargonic acid (Thinex), endothalic acid (Endothal), ammonium thiosulfate (ATS), and Torgitol TMN-6 , applied at different stages of bloom on fruit set, fruit quality, and yield of different cultivars of apples, peaches, plums, and nectarines. We found that Tergitol TMN-6 at the rates of 0.75% to 1.25%, applied at 75 to 80% bloom had excellent results in blossom thinning of stone fruit and application at 1870.8 L.ha-1 was often more effective than at 935.4 L.ha-1. Tergitol TMN-6 also was an effective blossom thinner for ‘Rome Beauty’ apples. Lime sulfur and Fish oil and combination of these two chemicals were also used as organic blossom thinners for apples and peaches, but they were not as effective as Tergitol TMN-6. Dormex was an effective blossom thinner for apple and stone fruit and did not cause fruit russetting. Application of ATS and Wilthin were effective but results were variable from season-to season. Fruit size increased when effective blossom thinning occurred. Wilthin, at high concentrations, caused fruit marking in apples. The majority of thinning done in the humid apple growing regions of the world is with postbloom thinners. The most commonly-used postbloom thinners are naphathaleneacetic acid, carbaryl, benzyladenine, naphthaleneacetamide and, to a lesser extent, ethephon. More commonly, postbloom thinners are applied in combinations which may allow them to be applied at lower individual rates. Combinations may be applied alone at different stages of fruit development or at the same time as a tank mix. Greater fruit abscission following thinner applications is associated with weather conditions that favor a reduction in carbohydrate levels in trees, especially low light levels and elevated temperature after application.